1. Field of the Invention
This invention relates to an apparatus for reading radiation image information that is stored and recorded on a stimulable phosphor sheet.
2. Prior Art
Certain phosphors, when exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, electron beams and ultraviolet rays, store part of the energy the radiation possesses, when the phosphor is subsequently exposed to excitation light such as visible light, it produces stimulated emission corresponding to the stored energy. The phosphor exhibiting such properties is known as a stimulable phosphor.
The assignee has proposed a system for recording and reproducing radiation image information using the stimulable phosphor. In the system, the radiation image information of an object such as the human body is first stored in a sheet having a layer of stimulable phosphor (which is hereinafter sometimes referred to simply as a "phosphor sheet") and the sheet is then scanned two-dimensionally with excitation light such as laser light to produce stimulated emission. The stimulated emission is read photoelectrically to obtain image signals, on the basis of which a radiation image of the object is produced as a visible image on a recording material such as a photographic material or a display device such as CRT (see, for example, Japanese Patent Application Kokai Nos. 55-12429 and 56-11395).
In this system for recording and reproducing radiation image information, the information of interest is read from the phosphor sheet in the following manner by means of various types of apparatus for reading radiation image information (which apparatus is hereinafter sometimes referred to simply as a "reading apparatus"). Excitation light of a given intensity that is emitted from a light source such as a He-Ne laser is reflected and deflected in the main scanning direction by means of a light deflector such as a galvanometer mirror and is passed through various optical elements including an f.theta. lens to illuminate the phosphor sheet. The phosphor sheet is transported by a transport means such as a belt conveyor or nip rollers in the sub-scanning direction, or a direction generally perpendicular to the main scanning direction. Thus, the excitation light deflected in the main scanning direction is capable of two dimensional "blanket" scanning of the phosphor sheet.
The area of the phosphor sheet that is illuminated with excitation light produces stimulated emission in accordance with the radiation image information stored and recorded in that area. The stimulated emission is incident on the entrance face of an optical guide either directly or after being reflected by a cylindrical condenser mirror that is located in a face-to-face relationship with that entrance face. The incident stimulated emission travels as guided by the optical guide and, after passing through a filter that cuts off light in the wavelength range of excitation light, the stimulated emission is launched into a photo multiplier where it is converted electric signals. After appropriate processing, the signals are reproduced as a visible image on a CRT or photographic material or recorded for storage in various recording media including optical disks
One of the problems associated with this reading apparatus that reduce the precision of image reading is flare. While there are several types of flare including unwanted external light, the most significant in the reading apparatus is excitation light that is reflected or scattered after being incident on the phosphor sheet A as indicated by one-short-and-one-long dashed lines in FIG. 16. Part of this flare is reflected or scattered by the cylindrical condenser mirror 106 or the entrance face 104 of the optical guide 102 to make a second and direct entry into the phosphor sheet A. Another part of the flare denoted by 100a or 100b is further reflected or scattered by the entrance face 104 of the optical guide 102 or the cylindrical condenser mirror 106 to be incident again on the phosphor sheet A. Still another part which is denoted by 100c travels back in a direction generally opposite to the excitation light L (upward in the case shown in FIG. 16) to be reflected or scattered by the housing of the scanning optical unit or the optical elements in it, whereupon the flare is again incident on the phosphor sheet A. Whichever of these types of flare will excite the stimulable phosphor to produce stimulated emission.
The stimulated emission produced by this flare, as well as the stimulated emission produced by excitation light L will be launched into the optical guide 102 and transmitted therethrough to be read as image information However, it is very rare that the flare is incident on the image reading position (scanning line) 108, or the position identical to that of illumination with excitation light L. In other words, most of the stimulated emission produced by the flare comes from a different position than where the intended information should be read. As a consequence, the eventually read image will have a lower contrast, interfering with the reading of correct image information. Further, if the image of lower contrast is processed electrically, the image information associated with the flare will be amplified as noise, thereby making it impossible to reproduce a satisfactory visible image on a CRT or various kinds of recording materials.
In order to solve these problems, the assignee has proposed the following various reading apparatuses: an apparatus having a dichroic coating on the cylindrical reflecting face 110 of the cylindrical condenser mirror 106 that reflects stimulated emission but which does not reflect the excitation light (Japanese Patent Application Kokai No. 60-189736); an apparatus having a screen on the entrance face 104 of the optical guide 102 to prevent the reflection of excitation light (Japanese Patent Application Kokai No. 60-189737); an apparatus that has a filter provided between the scanning line 108 and the cylindrical condenser mirror 106 and/or the entrance face 104 of the optical guide 102, which filter absorbs excitation light but transmits stimulated emission (Japanese Patent Application Kokai No. 61-128239); an apparatus that is an improvement over the apparatuses disclosed in Japanese Patent Application Kokai Nos. 60-189736 and 60-189737 in that a single mirror having a dichroic coating is provided on the back side of the cylindrical condenser mirror so that the excitation light passing through the cylindrical condenser mirror is reflected by means of the added mirror towards the position in which the reading of image information from the phosphor sheet has ended, whereby the excitation light is also used as erasure light (Japanese Patent Application Kokai No. 61-65231).
The assignee also proposed in Japanese Patent Application Kokai No. 60-46166 and apparatus having a slit plate as shown by 112 in accompanying FIG. 17. The slit 112 is provided between the entrance face 104 of the optical guide 102 and the phosphor sheet A and has an aperture 114 of predetermined size corresponding to the scanning line 108. Flare reflected from the phosphor sheet A as typically denoted by 100d is blocked by the slit plate 112 and will not make reentry into the phosphor sheet A.
In the reading apparatuses described above, the flare that is reflected by either the cylindrical reflecting face 110 of the cylindrical condenser mirror 106 or the entrance face 104 of the optical guide 102 or both to be incident again on the phosphor sheet A can be substantially reduced to enable radiation image information to be read with much higher precision than the previous versions.
However, in the apparatus disclosed in Japanese Patent Application Kokai No. 60-189736, flare of small intensity can occur as a result of light transmission through the cylindrical reflecting face 110 and subsequent reflection by a support member for the cylindrical condenser mirror 106 (as indicated by dashed lines in FIG. 18) and this flare cannot be eliminated from the apparatus. In the apparatus disclosed in Japanese Patent Application Kokai No. 60-189737, the optical guide 102 is usually molded of plastic materials such as acrylic resins and this makes it difficult to provide a dichroic coating of high precision. Furthermore, flare cannot be totally eliminated from this apparatus. In the apparatus disclosed in Japanese Patent Application Kokai No. 61-128239, the need to provide a filter in the neighborhood of the scanning line lowers the degree of freedom in design. Further, the need to use the filter and an associated support member increases the complexity of the apparatus.
The apparatus disclosed in Japanese Patent Application Kokai No. 61-65231 has the problem that mounting a dichroic mirror at a specified angle of inclination on the back side of a dichroic cylindrical condenser mirror is extremely difficult to achieve while at the same time the degree of freedom in design is lowered. Further, it is difficult to insure that the flare incident on the cylindrical condenser mirror is directed exclusively towards the area of the phosphor sheet where information reading has ended. It is also difficult to achieve complete prevention of entrance into the optical guide of the stimulated emission produced by the flare. In order to meet these needs, still another member must be added but then the complexity of the apparatus is increased.
Under these circumstances, it has been desired to develop an apparatus for reading radiation image information that is capable of reducing the flare that originates from a cylindrical condenser mirror.
The conventional apparatuses described above have the additional problem that flare as typically denoted by 100c that travels back in a direction opposite to the excitation light and that is reflected by the housing of the scanning optical unit and associated optical elements to make a second entry into the stimulable phosphor sheet cannot be reduced. Hence, those apparatuses need a further improvement to reduce the flare of the type described above.
The apparatus disclosed in Japanese Patent Application Kokai No. 60-46166 has the light-shielding slit plate 112 disposed between the phosphor sheet A and each of the cylindrical condenser mirror 106 and the optical guide 102 as depicted in FIG. 17. The major problem with this apparatus is that in the presence of the slit plate 112, the stimulated emission that is supposed to be launched into the optical guide 102 is blocked as indicated by dashed lines in FIG. 17. This reduces the quantity of stimulated emission that is effectively launched into the optical guide 102, causing a substantial decrease in the efficiency of light collection. Another problem is that if the phosphor sheet A flexes, it will contact the slit plate 112, potentially causing unevenness in the speed of transport in the sub-scanning direction. If the flex of the phosphor sheet A is significant, even jamming can occur.
As shown in FIG. 16, part of the flare that is launched into the optical guide 102 through the entrance face 104 is reflected by an adhesive layer at the exit face of the optical guide 102 and emerges from the entrance face 104 to be reentrant into the phosphor sheet A. A side view of the light collecting unit in the reading system under consideration is shown schematically in FIG. 19. The optical guide 102 is formed of a plastic material such as an acrylic resin. The exit face 120 of the optical guide 102 is fitted with a color filter 124 for absorbing excitation light L, with a light-transmissive layer 122 being interposed. A photo multiplier 128 which serves as a photodetector is bonded to the color filter 124 with a light-transmissive adhesive layer 126 being interposed. Each of the optical guide 102 (stated more exactly, the material of which it is made) and the filter 124 has such a great difference in refractive index from the adhesive layer 122 that part of the flare that is launched into the optical guide 102 at the entrance face 104 to be guided through it together with the stimulated emission is reflected either at the interface between the adhesive layer 122 and the exit face 120 of the optical guide or at the interface between the adhesive layer 122 and the filter 124, whereupon the flare travels back in a direction opposite to that of incidence and emerges from the entrance face 104 to make a second entry into the phosphor sheet A, producing undesirable stimulated emission from the latter.
Hence, the conventional apparatuses described above have the yet another problem that they are incapable of eliminating the flare that is first launched into the optical guide 102 and which is then reflected at the interface between the adhesive layer 122 and each of the exit face 120 of the optical guide and the filter 124 to make a second entry into the phosphor sheet A. Under these circumstances, it has been desired to develop an apparatus for reading radiation image information that is further improved to reduce the adverse effects of flare of the kind described above.